1. Field of the Invention
This invention generally relates to a novel magnetic refrigerant or magnetic refrigeration working substance for use in magnetic refrigerator, and particularly, to a magnetic refrigerant having an amorphous structure and to processes for producing the same.
The magnetic refrigerator utilizes a magnetic calorie effect of the magnetic refrigerant and has an advantage of its high cooling capability per unit volume, as compared with a gas refrigerator and hence, it is used in the production of liquid helium.
Magnetic refrigeration is based on the principle of alternate repetition of two heat-exchange steps: a heat exhausting step of magnetizing the magnetic refrigerant, wherein heat generated thereby is released to the outside, and a heat absorbing step of abstracting heat from an object such as helium by the magnetic refrigerant cooled by adiabetic demagnetization. In the case of Ericsson cycle as a refrigeration cycle, a work W performed by a magnetic material is represented by W=.DELTA.S.sub.M (T.sub.1 -T.sub.2), wherein .DELTA.S.sub.M is a magnetic entropy; T.sub.1 is a high temperature in the cycle; and T.sub.2 is a low temperature in the cycle. The magnetic refrigerant is required to have characteristic such as a large magnetization in the range of operation, a high coefficient of thermal conductivity in the range of operation, and be a large-sized block.
In general, the magnetic refrigerant is classified broadly into a type used in a range of low temperature of less than 20 K., and a type used in a range of high temperature of 20 K. or more. GGG (Gd.sub.3 Ga.sub.5 O.sub.12) belongs to the former, and compounds containing a rare earth element or elements belong to the latter. The magnetic refrigerant according to the present invention belongs to the latter.
2. Description of the Prior Art
There is a conventioanlly known magnetic refrigerant having an amorphous structure and containing a rare earth element or elements, as disclosed in Japanese Laid-open Patent Application No. 37945/86. This magnetic refrigerant is produced by a melting process such as a single-roll process, or by a spattering process.
However, a ribbon produced by the melting process usually has a thickness of 10 to 40 .mu.m and therefore, in order to produce a block larger in size than this ribbon, e.g., a thick plate, a large number of thin plates cut from a ribbon must be secondarily laminated and press-bonded to one another. However, the resulting thick plate has a problem in that each of the large number of thin plates contains an oxide film on their surface. Hence, the thick plate has a low coefficient of thermal conductivity, resulting in a reduced cooling efficiency.